This invention relates to a combusting device of the type comprising a perforated housing and encased therein tablets or pellets of a chemical composition which combusts with generation of a large quantity of gas, and more particularly to a buffer means in a device of this type for preventing breaking of the pellets before and during combustion. A device according to the invention can be embodied in a gas generator for an inflatable safety apparatus and an igniter for a rocket motor.
Some kinds of conventional safety or life-saving apparatus as exemplified by an inflatable safety bag for the protection of a car passenger from injuries in collision of the car are equipped with a gas generating device having a perforated housing and encased therein tablets or pellets of a chemical composition, usually a pyrotechnic composition, capable of producing a large quantity of gaseous reaction products in a short time upon combustion. In addition, a gas generator of this type includes a squib or initiator inserted into the housing to ignite the pyrotechnic pellets and usually contains a heat-absorbing material in the form of granules packed between the pyrotechnic pellets and the perforated wall of the housing to reduce the temperature of the reaction gas before discharge.
Also, many igniters for solid propellant rocket motors are constructed fundamentally similarly to the aforementioned gas generator except that the heat-absorbing material is excluded and that use is made of a pyrotechnic composition which generates a large amount of heat upon combustion as well as gaseous reaction products since the igniters are required to jet out a very high temperature gas or flame.
In a combusting device of the above described type, either in the case of a gas generator for a safety apparatus or in the case of an igniter for a rocket motor, the shape and dimensions of the tablets or pellets of the gas generating composition are determined carefully so as to realize an intended rate of combustion or gas generation at a suitable combustion pressure in the housing of the device. However, there is a possibility of cleaving or crushing of the pellets when the device is subjected to mechanical vibrations and/or shocks, for example, during carriage of the device on a car. If the pellets undergo combustion in a broken state, even partially, it is likely that the device will fail to exhibit an expected performance and that the combustion pressure will become excessively high because of an enlarged total surface area of the combustible pellets, resulting in rupture of the housing of the device in an extreme case.
Therefore, conventional combusting devices of the described type comprises certain means for holding the pellets of the gas generating composition in the housing practically motionless thereby to prevent breaking of the pellets. A typical example of such means is a spacer which is made of a relatively soft material such as a plastic foam or glass wool and interposed between the pellets and a non-perforated wall of the housing such that the pellets are packed closely in the housing under a compressive load. However, it is difficult to completely prevent movements of the pellets by the provision of such a spacer. Even when the pellets are kept unbroken before actuation of the device, the spacer will be forced to decrease its volume, causing an enlargement of the volume of a combustion chamber in which are disposed the pellets, as the pressure in the housing rises upon initiation of the combustion of the pellets. Then the pellets under combustion begin to move around in the combustion chamber and collide against each other, so that there occurs cleaving or crushing of a portion of the pellets before completion of combustion. Besides, the use of the spacer brings about an inconvenience to the assemblage of the combusting device. To dispose the spacer as designed, the assemblage of the device must be performed either by charging the housing with the combustible pellets after installation of the initiator to the housing or by inserting the initiator into the housing in a state packed with the pellets, so that there is the danger of an accidental firing of the initiator by collision or friction between the initiator and the pellets.
As an alternative technique employed in some of conventional gas generators, the combustible pellets are packed closely in a bag of cloth, plastic film or metal foil, and springs are disposed in a space between the outside of the bag and the inside of a housing wall so as to hold down the bagged pellets with a compressive force. However, this method, too, is not effective for preventing the pellets from moving around and breaking during combustion since the springs are compressed as the pressure in the bag rises, causing an enlargement of a space in which are confined the pellets. Besides, it is very troublesome to assemble the device using this method because of the need of closely packing the pellets in the bag without causing the bag to swell out beyond the inner diameter of the housing and thereafter inserting the bagged pellets into the housing without damaging the bag which is made of a thin and soft sheet material.